The island nation of Sri Lanka is separated from southeastern India by the Palk Strait, a 40-mile-wide stretch of sea named for Robert Palk, a governor of Madras in British India. But look closer on a map: The two countries are more closely connected than you might think. A chain of coral islands and sandy shoals twists across the strait between India's Pamban Island and Sri Lanka's Mannar Island. It's called Adam's Bridge, and as recently as 500 years ago, you could walk across it.

Until the 15th century, Sri Lanka was a peninsula, not an island.

By India's official count, Adam's Bridge consists of 103 tiny reefs and sandbanks that curve gracefully toward the northwestern tip of Sri Lanka. At low tide, many emerge from the waves, and the annals of a nearby temple say that the chain was an unbroken causeway until it was breached by a fierce tropical storm in 1480.

For Muslims, Adam's Bridge is a giant's causeway.

The bridge took its name from an Islamic tradition that the Garden of Eden was located in Sri Lanka. Sri Pada, a Sri Lankan mountain with a footprint-shaped mark at its peak, is often called Adam's Peak, and is identified with the place where the first man fell to Earth after his expulsion from Eden. (In several Muslim hadiths, Adam is described as standing 60 cubits tall—over 90 feet!—which explains his giant footprint.) Adam's Bridge, then, was the path he took as he journeyed to mainland Asia.

For Hindus, the bridge was built by a monkey army.

Scientists aren't sure what formed this unusual ribbon of shoals. Is it a ridge formed by a thinning of the earth's crust in the Palk Strait? A "tombolo" of sand deposited by ocean currents? Remnants of an ancient coral shoreline from before Sri Lanka separated from India? One ancient Sanskrit epic poem provides a more colorful answer. In the sixth book of the Ramayana, the god Rama and his monkey companions engineer a floating bridge to the kingdom of Lanka, so Rama can rescue his beloved Sita from the demon-king Ravana. As a result, Indians call Adam's Bridge "Rama Sethu"—Rama's Bridge. The Indian end of Rama Sethu, near the town of Rameswaram, is one of the holiest places in Hinduism and a popular pilgrimage site.

Five hundred years later, Adam's Bridge could become a real bridge again.

The Indian sand spit where Adam's Bridge begins was once the thriving port city of Dhanushkhodi, but it's been a ghost town since a devastating 1964 storm that killed 1,800 people in the region. But Dhanushkhodi could rise again: In June, India revived a centuries-old project to build a road bridge across the Palk Strait at Adam's Bridge. Some in Sri Lanka are wary of the plan, though. Indian political influence is one worry, but so is malaria. The deadly disease has been virtually eliminated in Sri Lanka, but India still has more than a million cases a year, and Sri Lanka doesn't want them crossing the strait as easily as Rama or Adam did.

Explore the world's oddities every week with Ken Jennings, and check out his book Maphead for more geography trivia.